The present invention relates in general to improvements in nickel-based superalloys and more particularly to compositions and methods for improving the creep resistance of such alloys at specific preselected temperatures.
Exemplary of nickel-based superalloys is alloy 718 which has a composition specification, according to the Society of Automotive Engineering and Aerospace Material Specification AMS5662E of 50-55 wt % Ni, 17-21 wt % Cr, 4.75-5.50 wt. % Nb+Ta, 2.8-3.3 wt % Mo, 0.65-1.15 wt % Ti, 0.2-0.8 wt % Al, 0.35 wt % Mn (max.), 0.08 wt % C (max), 0.015 wt % S (max), 0.015 wt % phosphorus (max), 0.015 wt % Si (max), 1.00 wt % Co (max), 0.006 wt % boron (max), 0.30 wt % Cu (max), with the balance Fe.
The nominal composition of the alloy is 53 wt % Ni, 18.0 wt % Cr, 18.5 wt % FE, 5.2 wt % Nb (and Ta), 3.0 wt % Mo, 1.00 wt % Ti, 0.50 wt % Al, 0.04 wt % carbon, and 0.004 wt % boron with phosphorus in the range of 0.005-0.009 wt % or 50-90 ppm. This alloy is a precipitation hardened nickel-base alloy with excellent strength, ductility and toughness throughout the temperature range xe2x88x92423xc2x0 F. to +1300xc2x0 F. The alloy is normally provided in both cast and wrought forms and typical end use parts, such as, blades, discs, cases and fasteners are characterized by high resistance to creep deformation at temperatures up to 1300xc2x0 F. (705xc2x0 C.) and by oxidation resistance up to 1800xc2x0 F. (908xc2x0 C.). In particular, parts which are formed or welded and then precipitation hardened develop the desired properties. These properties, along with oxidation resistance, good weldability and formability, account for its wide use in aerospace, nuclear and commercial applications.
It is well known, as in U.S. Pat. No. 3,660,177, that the fatigue resistant properties of the alloy can be substantially improved by adjusting the processing practice in ways that promote the formation of ultra fine grain size. Unfortunately, the formation of ultra fine grain size and its beneficial effect on fatigue properties is accompanied by an unwanted reduction in stress rupture properties or creep resistance at preselected test temperatures. It is therefore desirable to provide an improved alloy which exhibits better stress rupture life while maintaining a constant ultra-fine grain size and therefore fatigue resistance comparable to conventional 718 alloy.
An objective of the present invention is to improve the creep resistance of nickel-based alloys while maintaining a constant ultra-fine grain size and other desired properties, such as fatigue resistance.
The stress rupture life of fine-grained nickel-based alloys is improved at certain temperatures and stresses by the synergistic effect of predetermined amounts of phosphorus (P) and boron (B) in the alloy composition and more particularly in such alloys having low carbon content.
The element boron by itself, or in combination with zirconium has in the past been purposely added to nickel-based alloys for the purpose of improving stress rupture and creep properties. Phosphorus, on the other hand, is considered a xe2x80x9ctrampxe2x80x9d elementxe2x80x94that is, it is not purposely added, but carried in as a contaminant with various raw materials used to produce nickel-based alloys and has generally been considered as detrimental to properties if the content is allowed to exceed very low limits. Most commercial specifications for nickel-based alloys place a low maximum limit on phosphorus content. Specification AMS 5662E, for example, restricts phosphorus to 0.015% maximum.
It has been discovered however, that purposeful additions of phosphorus, even in excess of the nominal commercial specification limits, can surprisingly improve the stress rupture properties of certain nickel-base superalloys by as much as an order of magnitude (10xc3x97) or 1000%.
It has further been discovered that specific amounts of phosphorus, boron, and carbon in nickel-base alloys work together in a synergistic manner and that when all three elements are present in specific, controlled amounts, that even greater improvements in stress rupture properties can be obtained. These results are obtained with values that are more than additive of the results expected of each element individually. This synergistic effect is achieved while maintaining other desired properties such as tensile strength and fatigue resistance.
The desired effect of phosphorus and boron on stress rupture or creep deformation of superalloys according to the invention described herein, can best be understood from the following discussion. The controlling mechanism of creep deformation in most applications in nickel-based superalloys, particularly the alloys described herein, is dislocation creep which can occur at grain boundaries and the interior of the grains. Phosphorus and boron in nickel-based alloys have a strong tendency to segregate to grain boundaries and also remain inside the grains as solute atoms or as compounds (phosphides or borides), particularly when the grain boundaries are heavily occupied by phosphorus or boron. Usually phosphorus and boron will compete with each other for available grain boundary sites and phosphorus in this side competition has a stronger tendency to grain boundary segregation. At lower test temperatures, as described herein, transgranular dislocation creep dominates. Phosphorus and boron which remain in the interior of grains can retard creep deformation by their interaction with dislocations through several possible mechanisms, and a strong synergistic effect of phosphorus and boron on dislocation creep was observed, as more fully described hereinafter. However, phosphorus and boron which segregate to grain boundaries will not play any important role in retarding the transgranular dislocation creep. This may explain the lack of any observed effect of boron at low levels in alloys with ultra low phosphorus. That is, boron preferentially segregates to the grain boundaries, due to lack of site competition from phosphorus.
The synergistic effect described and the roles of varying amounts of phosphorus, boron and carbon in nickel-based alloys in improving stress rupture properties without detrimentally affecting fatigue life was characterized in the results of a systematic series of comparison tests described hereinafter.